This invention relates to a long time exposure preventing system in a focal plane shutter camera equipped with a flash generating device (hereinafter referred to as "an electronic flash" when applicable). It is particularly applicable in cameras in which the electronic flash is controlled by a light measurement circuit in the camera.
If the distance between an object and a camera varies in comprising and taking a picture of that object with an electronic flash, it is necessary to adjust or change the aperture value in accordance with the varied distance. This is because the intensity of light reflected by the object will also vary and theshutter speed is usually fixed.
In order to eliminate this inconvenience, in this technology the concept of automatic electronic flash has been proposed. In an automatic electronic flash, an electronic flash is provided with a light measurement circuit. The light measurement circuit operates to measure the quantity of light reflected by the object when the electronic flash emits a flash, and to stop the flashing operation of the electronic flash when the quantity of light measured reaches a correct exposure value. Hence a picture can be taken with the correct exposure at all times.
In general, electric-eye (EE) cameras are provided with light measurement circuits. When a picture is taken with such a camera with the automatic electronic flash described above, the light measurement circuit in the camera is not used. That is, it is in idle state and the camera is used in a manual mode of operation.
Furthermore, recently a circuit has been developed in which the flashing operation of an electronic flash is controlled by using a light measurement circuit incorporated in an electronic focal plane shutter camera. In this case a correct exposure can be readily obtained in taking a picture with an electronic flash without incorporating a light measurement circuit in the electronic flash. With this type of circuit, when the shutter top curtain is allowed to run by shutter release and the light photographing optical path is opened, the "X" contact on the camera is closed to permit the electronic flash to emit a flash. When the quantity of light reflected by the object reaches a correct exposure value, a shutter closing signal is generated by the light measurement circuit, energization of the shutter bottom curtain magnet is suspended, and the shutter bottom curtain is allowed to run. The shutter closing signal is also employed as a signal for stopping the flashing operation of the electronic flash, so that a picture is taken with the correct exposure.
However, since the quantity of light emitted by the electronic flash is limited, the correct exposure is not always obtained for the aperture value set in the camera. For instance, when a film having a sensitivity of ASA 100 is used, a correct exposure cannot be obtained with an electronic flash guide number "32" and an aperture value "F4"; that is, a picture will be underexposed. In this situation, since the correct exposure cannot be obtained, no shutter closing signal is generated by the light measurement circuit in the electronic focal plane shutter camera and as a result the bottom curtain controlling magnet is maintained energized. Therefore, the bottom curtain will not run. In other words, although the electronic flash has been operated to emit a flash of maximum duration, the exposure itself runs for a longer period of time. Such action will undoubtedly lead to poor picture quality due to camera shaking, and may lead to faulty operation and trouble of the camera.